Method for inoculating molten metal with an inoculating material

ABSTRACT

This invention describes a process by which an inoculating material, such as solid magnesium, can be introduced into a volume of molten metal, such as ferrosilicon alloy, so that the inoculating material can be efficiently alloyed into the molten metal with a minimum loss by vaporization and oxidation. The method involves bringing an ingot of inoculating material coated in a certain way into submerged contact with molten metal. The ingot is covered with a layer of ceramic insulating material except for a small area. When submerged in molten metal the ingot starts to melt where it is unprotected by the ceramic layer and as melting proceeds the face of the ingot material that is in contact with the molten metal proceeds into the ingot until the entire ingot is consumed. Thus the rate of melting the inoculating ingot is controlled.

United States Patent [191 Mezoff 1 Dec. 17, 1974 METHOD FOR INOCULATING MOLTEN Primary Examiner-Charles N. Lovell METAL WITH AN INOCULATING Assistant Examiner-Peter D. Rosenberg MATERIAL Attorney, Agent, or Firm-Head & Johnson [75] Inventor: John G. Mezoff, Tulsa, Okla. [57] ABSTRACT Assigneei American Magnesium p y This invention describes a process by which an inocu- Tulsa, Okla. lating material, such as solid magnesium, can be intro- [22] Filed; 17 1972 duced into a volume of molten metal, such as ferrosili- Appl. No.: 244,611

con alloy, so that the inoculating material can be efficiently alloyed into the molten metal with a minimum loss by vaporization and oxidation. The method involves bringing an ingot of inoculating material coated in a certain way into submerged contact with molten metal. The ingot is covered with a layer of ceramic insulating material except for a small area. When submerged in molten metal the ingot starts to melt where it is unprotected by the ceramic layer and as melting proceeds the face of the ingot material that is in contact with the molten metal proceeds into the ingot until the entire ingot is consumed. Thus the rate of melting the inoculating ingot is controlled.

4 Claims, 5 Drawing Figures BACKGROUND OF THE INVENTION In the prior art is has been well known that the introduction of a volatile metal, such as magnesium, into a molten mass of metal, such as ferrosilicon, will produce a magnesium-ferrosilicon alloy useful for alloying magnesium into iron to make iron strong and ductile. A principle problem is the introduction of the magnesium to be alloyed. These are three methos which are most commonly used for the production of such magnesium ferrosilicon alloys:

The first is to melt the required amount of magnesium in an induction furnace. To the molten magnesium is added molten 75 percent ferrosilicon (75 percent silicon and 25 percent iron). Even through a magnesium ferrosilicon alloy based upon 50 percent ferrosilicon is desired, it has been found necessary to first introduce 75 percent ferrosilicon to the melted magnesium because of the violence of the reaction between magnesium and ferrosilicon relates directly to the amount of iron present. After the introduction of some 75 percent ferrosilicon has occurred, it becomes practical to finish the heat by using 50 percent ferrosilicon.

The second method is the so-called duplex method. In this case the cold magnesium ingots are placed in the ladle and liquid silicon metal and 75 percent ferrosilicon are tapped onto the magnesium. The heat is finished by adding the required amount of 50 percent ferrosilicon. In order to homogenize the melt it is sometimes a practice to reladle it from one ladle to another to provide a thorough mixing.

The third method involves the plunging of an ingot of solid magnesium into the molten ferrosilicon melt.

All of the known methods have certain problems and deficiencies. It has been observed that the efficiency of the magnesium reaction (that is, the percent of magnesium actually retained in the solid alloy as compared with the percent magnesium which would be expected on a theoretical basis) relates to a number of factors. Among these are the ratio of surface area of the magnesium exposed to the melt to the mass of the magnesium, the depth below the surface of the melt to which the magnesium is plunged, the amount of iron present in the ferrosilicon alloy, and the temperature of the molten ferrosilicon.

It is known that when a solid piece of magnesium is immersed in a liquid ferrosilicon, the surface of the magnesium quickly reaches its melting point and the resulting liquid magnesium is quickly superheated to its boiling point. The resulting magnesium vapors perco- Iate upward through the ferrosilicon and react therewith to form the magnesium ferrosilicon alloy. The heat of fusion and heat of vaporization, which are both en dothermic, tend to have a moderating influence on the rate of melting of the solid magnesium.

When all of the variables are properly controlled in a production operation it has been possible to achieve a magnesium efficiency in the range of 75-85 percent. Further improvement of the magnesium efficiency is possible by reducing the rate at which the magnesium vapor is formed so that smaller bubbles of vapor with shorter reaction times are formed.

It is desirable to assure that complete reaction of a bubble of magnesium vapor occurs before the bubble can reach the surface'of the melt and escape to the atmosphere. The efficiency can be improved by assuring that vaporization occurs always at the lowest point possible in the melt. It has also been seen that some loss of magnesium occurs prematurely by reaction with the slag on the surface of the molten ferrosilicon at the point of introduction of the magnesium, when the plunging technique is used.

SUMMARY OF THE INVENTION It is a primary object of this invention to provide a method of introducing and alloying and inoculating material, such as magnesium into a melt of metal, such as ferrosilicon, in a more efficient and effective manner.

This object is accomplished in this invention by the application of a refractory, thermal insulating coating to the surface of a piece of inoculating material, such as for the purpose of preventing the melting of all surfaces so coated. A part of the surface of the piece of inoculating material is either left uncoated in a selected location or locations, or the coating is purposely removed at said selected locations, to permit the reaction of the inoculating material with the melt to occur. Such a partially coated piece of material is immersed into the melt to permit the reaction to take place at a controlled rate preferably at the lowest point in the melt. The coating also provides a physical barrier on the surface of the inoculating material to prevent its coming in contact with surface slag and thus cause premature reaction therewith and loss of inoculating material.

This and other objects of the invention and a better understanding of the principles and details of the invention will be evident from the following description taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a vertical section through a ladle of molten metal indicating the application of inoculating material described in this invention.

FIGS. 2, 3, 4 and 5 indicate the progress of melting of the ingot of inoculating material as a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention is concerned with a method of inoculating molten metal with an inoculating material. As an example of the application of the invention it will be described as it relates specifically to the inoculation of molten ferrosilicon with magnesium to produce a magnesium ferrosilicon alloy commonly used to alloy magnesium with iron to make ductile iron, it being understood that such use is merely exemplary of an application of the invention and is not intended to limit the invention. I

Referring to the drawings, numeral 10 indicates the ladle which is partially filled with molten ferrosilicon 12. The ferrosilicon would typically be at a temperature offrom 2,600 to 2,800 F. An ingot of magnesium 14 which has been fastened to a support rod 16 has been lowered into the molten ferrosilicon. The support rod 16 passes through the cover 18 of the ladle. Since ingot 14 has a specific gravity less than the ferrosilicon 12 means must be provided to hold the ingot submerged. This is accomplished by way of example, by clamping rod 16 to cover 18, such as by means of a set screw 22. Thus the weight of cover 18 serves to keep ingot 14 submerged.

Prior to the introduction of the magnesium into the ladle, it is coated except for a small portion thereof, with a ceramic thermal insulating layer 22. This ceramic layer provides a lag in the heating of the magnesium ingot. However, in the area such as 30 where the coating has been left off, or has been removed, the magnesium is in direct contact with the molten ferrosilicon. The magnesium is heated thereby to the melting and boiling point so that vapors of magnesium will filter upward through the molten metal and be in intimate contact with the ferrosilicon and will alloy with it. The smaller the bubbles of magnesium the greater will be the bubble surface area in contact with molten ferrosilicon and the greater the chance that they will be completely reacted and alloyed before they pass through the surface and escape into the atmosphere through the openings 26 in the cover plate 18.

It is important therefore to control the rate at which the magnesium is melted and vaporized so as to permit efficient alloying of the magnesium vapor with the ferrosilicon. Thermal insulation therefore provides means of protecting the surface of the ingot except where it is intended that the melting will take place.

The coating may be one of a group of materials comprising commercially available semi-silica refractory washes commonly available, or it may be made by mixing together refractory powders such as magnesium oxide, talc, iron oxide, etc., with water and a binding agent such as sodium silicate. In the presentation of the coating material sufficient water is added to permit appropriate coating by dipping the assembly into a bath of the wash followed by air drying. To build up the desired thickness of coating it may be found expedient to apply additional layers after air drying. It is also possible to brush or spray such coatings on to the assembly. One method for exposing the bare ingot surface is to mask that area using ordinary commercial masking tape prior to dipping, spraying, or brushing and to remove the tape after the coating has been applied.

FIGS. 2, 3, 4, and show, as a function of time, the progress of the melting of the magnesium ingot 14. At the start, the bottom, end, sides and top of the ingot are covered except for one or more small areas24 near the lower end of the ingot. As melting occurs, these areas 24 expand inwardly and later are enlarged and as time goes on the surface 24 moves upward, until the entire volume of magnesium has been melted. The bottom surface is preferably left covered as illustrated so as to minimize contact of the magnesium with the slag on top of the melt when the ingot is introduced into the ladle. This prevents a loss of expensive magnesium that might otherwise be alloyed with the slag.

The support rod 16 may also be coated so that it will not be melted by contact with the molten ferrosilicon. However, a refractory material can be used for the support rod, which need not be coated since it will not melt at the temperature of the ladle, and can thus be used over and over again to introduce new ingots of magnesium. I

lt will be clear also that a plurality of ingots similarly attached to support rods could be used. These ingots can be spaced horizontally from each other within the melt, so that as the magnesium vapor rises upward, vapor from each of the separate ingots will be contacting different parts of the melt of ferrosilicon, and the alloying will be done with fresh ferrosilicon metal. By the use of multiple ingots in this way the rate at which the magnesium can be introduced can be further controlled, the alloying will be more effective, and a minimum amount of magnesium vapor lost to the atmosphere. Q r

It has been found that through the use of the coating the rate of reaction can be moderated very effectively, making it practical to immerse very large ingots, thus eliminating the extra cost of consecutive immersing of more than one uncoated ingot, as is commonly practiced in production.

Moreover, immersing single large coated ingots makes it possible to reduce the total production time required for the inoculating operation, and thus to start inoculation at lower ferrosilicon temperatures. Magnesium recovery is enhanced when the ferrosilicon temperature is nearer 2,600 F than 2,800 F at the start of inoculation.

The ability to moderate and control the reaction rate when practicing the invention makes it possible to produce certain magnesium ferrosilicon alloys which normally are too reactive to be made in conventional equipment. Many ductile iron producers prefer a lower silicon content and a higher iron content in the alloy.

It is now possible through the use of the invention to produce magnesium ferrosilicon alloys containing less than 50 percent silicon.

In addition to the method illustrated in which the coated ingot in plunged into molten metal, the invention may be practiced by tapping-on molten metal into a vessel containing the coated ingot or ingots. Conventional methods may be utilized to keep the ingot or ingots submerged in the molten metal. This may be accomplished such as by placing the ingot or ingots under a ledge or in a vented cage of refractory material. Thus the invention is not limited to a specific means by which the coated ingot or ingots are brought into submerged contact with molten metal.

To illustrate one advantage of the invention a 5,480 pound melt of ferrosilicon containing 48 percent silicon and the normal content of rare earths and calcium was inoculated with 420 pounds of magnesium, following normal practice. The magnesium content of the resulting magnesium ferrosilicon alloy was 5.54 percent; of magnesium recovery of 77.81 percent.

An identical melt of ferrosilicon was then inoculated with 420 pounds of magnesium by the use of selectiveiy coated ingots as described herein. The magnesium content of the resulting magnesium ferrosilicon alloy was 6.52 percent; of magnesium recovery of 91.57 percent.

While the invention has been described with a certain degree of particularity it is manifest that many changes may be made in the details of construction and the arrangement of components. It is understood that the invention is not to be limited to the specific embodiments set forth herein by way of exemplifying the invention, but the invention is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which'each element or step thereof is entitled.

What is claimed is:

l. The method of inoculating a volume of molten ferrosilicon with magnesium at a controlled rate, comprising the steps of:

6 coating substantially all of the exterior surface of an mersing the coated ingot in the molten metal includes:

i'ngot of magnesium with'a ceramic thermal insulating material except for a small selected uncoated attaching a support rod to the ingot of inoculating area; material; and

immersing the coated ingot in the said molten ferro- 5 immersing the rod and ingot into the molten metal.

silicon metal.

2. The method as claimed in claim 1 in which the 4. The method as in claim 1 in which the step of imcoating of said ingot is of sufficient thickness to provide mersing the coated ingot in the molten metal includes: sufficient insulation to prevent the vaporization of said inoculating material under the coating until the melting 0 placing the coated ingot in a vessel; and pouring the has progressed from said uncoated area. molten metal into the vessel.

3. The method as in claim 1 in which the step of im- 

1. THE METHOD OF INOCULATING A VOLUME OF MOLTEN FERROSILION WITH MAGNESIUM AT A CONTROLLED RATE, COMPRISING THE STEPS OF: COATING SUBSTANTIALLY ALL OF THE EXTERIOR SURFACE OF AN INGOT OF MAGNESIUM WITH A CERAMIC THERMAL INSULATING MATERIAL EXCEPT FOR A SMALL SELECTED UNCOATED AREA; IMMERSING THE COATED INGOT IN THE SAID MOLTEN FERROSILICON METAL.
 2. The method as claimed in claim 1 in which the coating of said ingot is of sufficient thickness to provide sufficient insulation to prevent the vaporization of said inoculating material under the coating until the melting has progressed from said uncoated area.
 3. The method as in claim 1 in which the step of immersing the coated ingot in the molten metal includes: attaching a support rod to the ingot of inoculating material; and immersing the rod and ingot into the molten metal.
 4. The method as in claim 1 in which the step of immersing the coated ingot in the molten metal includes: placing the coated ingot in a vessel; and pouring the molten metal into the vessel. 